Coesite atomic dynamics revisited: combining in situ Raman spectroscopy under non-ambient conditions and ab initio calculations

Abstract

Natural coesite was experimentally studied up to ∼9 GPa at room temperature and up to 1373 K at ambient pressure by in situ Raman spectroscopy and the phonon pressure evolution was modelled by ab initio calculations. The high-pressure results show that phonon compressibility of several low-energy modes changes above 6 GPa. Phonon assignment of detected Raman active modes has been carried out by comparing the measured and calculated wavenumbers as well as their pressure derivatives. We show that the experimentally observed modulations of the phonon compressibilities reflect the way atomic displacement vectors of such phonon modes reorient due to structural changes occurring in coesite with increasing pressure. High-temperature experiments of coesite show that most low-wavenumber phonon modes (below 500 cm−1) soften following a non-linear (parabolic) trend with a significant decrement in its temperature derivative above ∼900 K. As coesite is expected to undergo congruent melting above 800-900 K, we suggest that these anomalies may reflect the response of atomic dynamics to the thermodynamic metastability of coesite under such overheated conditions. Finally, the mode Grüneisen parameters of the most intense coesite Raman peaks were calculated by involving both the high-pressure and the high-temperature measurements. At ambient conditions, the low-energy modes show extremely large isobaric Grüneisen parameters (γP) (derived from high-temperature datasets), considerably exceeding the isothermal values (γT) (derived from high-pressure datasets). Consequently, the intrinsic anharmonicity of coesite is very high at ambient conditions. However, we show that high temperature significantly reduces γP while high pressure slightly increases γT of most phonon modes. Combined high-pressure and high-temperature experiments are therefore necessary to better understand the correlation between the coesite intrinsic anharmonicity and its thermodynamic stability.